In the 19th century, numerous still-picture cameras were created for taking panoramic still pictures through split-rotating mechanisms. Straight forward, yet tedious, procedures were used to first capture multiple still pictures of a surrounding scene and then stitch the pictures together as a seamless panoramic image. Even today, these types of cameras utilize mechanical moving parts and need tedious manual procedures to process multiple pictures in a panoramic view. Ultimately, these types of cameras are inherently awkward and cannot be used to generate wide field-of-view real-time video. The term “wide angle field-of-view” may vary for different applications. For example, for automobile rearview mirrors, a wide angle field-of-view may range between 30 degrees and 90 degrees. However, for other applications, such as surveillance systems, wide angle field-of-view may range between 100 degrees and 360 degrees. It should be understood that different applications may have generally understood fields-of-view that have industry accepted ranges, where a wide angle field-of-view is considered above a certain field-of-view value or within a range of values of fields-of-view.
With rapid advances in high-resolution charge-coupled device or complementary metal oxide semiconductor (CCD/CMOS) video sensor technology in recent years, much research went into developing video cameras and techniques that can simultaneously provide a 360° field-of-view. Resulting from the research and development of high-resolution CCD-CMOS video sensor technology, most 360° video cameras use conventional optics with fisheye lenses or omni-directional mirrors to obtain the 360° field-of-view. FIG. 1 shows a conventional wide angle video surveillance system 100 for capturing a scene up to a 360° field-of-view. Two alternative and different input elements are typically used for capturing a wide-angle field-of-view image of a scene, where the two different input elements include an omni-directional mirror capturing system 102a and a fisheye lens image capturing system 102b. These two different wide angle field-of-view image capturing systems 102a and 102b use different techniques for viewing a scene, but produce similar images. Each of the wide angle field-of-view image capturing systems 102a and 102b use CCD/CMOS image sensors 104a and 104b to collect video images of the scene. However, the wide angle field-of-view sensing system 102a uses an omni-directional lens 106 to reflect images of the scene onto the CCD/CMOS image sensor, while the wide angle field-of-view imaging system 102b uses a fisheye lens 108 to collect images of the scene and focus the images onto the CCD/CMOS image sensor 104b. Although each of these two systems 102a and 102b use different image capturing techniques (i.e., reflective omni-directional mirror versus fisheye lens), a resulting image of each of the two techniques is captured onto a CCD/CMOS image sensor chip that is typically rectangular.
A typically CCD/CMOS image sensor chip is formed of a pixel array having a 4:3 ratio of width to height. As shown, a CCD/CMOS image sensor chip 110 is formed of pixels, and an image is imaged onto the CCD/CMOS image sensor chip 110 in an image area 112 that is circular in nature. An image 114 within the image area 112 is shown to be highly distorted, circular, and not suitable for direct viewing. In addition, the image area 112 utilizes only approximately 58% of the pixels available on the CCD/CMOS image sensor chip 110, which means that approximately 42% of the pixels on the CCD/CMOS image sensor chip 110 are not used. This low rate of pixel utilization significantly deteriorates resolution of a final video output due to an entire image being compressed into a small area of pixels.
FIG. 2 is an illustration of an exemplary image sensor chip 200 that shows an image area 202 imaged onto the image sensor chip 200 as a result of a traditional fisheye lens or parabolic mirror that generates circular images. Because the image is circular, it uses a relatively small portion of effective pixels on the CCD/CMOS sensor chip 200. Because only a small portion of the pixels are used, resolution is compromised during video processing to unwrap the circular image in the image area 202 and enlarge the image to be displayed on a video display or storage on a storage medium, such as a computer disc or memory.
Continuing with FIG. 1, to facilitate visualization of the image 114 that is imaged onto the CCD/CMOS image sensor chip 110, a computing system 116 is typically used to digitally resample and unwrap the image 114 displayed in a circular image in the image area 112 to produce rectangular image in which the abscissa represents azimuth and ordinate elevation. An unwrapped and unwarped image 118 is shown to be output from the computing system 116. The computing system 116 uses complex software to perform the unwarping, unwrapping, and other correction of the image. However, because the digital unwarping requires external computation resources, a wide angle field-of-view system is increased in cost, complexity, and size. Because of these and other issues, wide angle field-of-view video cameras have not yet found wide-spread applications.
Fisheye lenses and reflective omni-directional mirrors are often used for wide angle field-of-view imaging systems because these optical elements are rotationally symmetric. Although a fisheye lens is conceptually relatively easy to build, fisheye lenses generally include multiple lenses. These multiple lenses or multistage lenses may introduce strong optical aberrations, which, in turn, need additional lenses to correct. As a result, fisheye lenses and other rotationally symmetric optical elements generally require more complex optics for an entire system, are bulky in size and weight, and expensive in cost. In general, wide angle field-of-view systems that use rotationally symmetric optical elements use computing systems to alter a sensed image to be properly viewed by a person (e.g., without being wrapped or warpped). In addition to the problem of low pixel utilization of a CCD/CMOS sensor chip, fisheye lenses also suffer severe axial unevenness of pixel resolution (i.e., a peripheral edge has much lower pixel resolution than in the center area). For most surveillance systems, the ability to detect targets in peripheral areas may be as or more important than in a central area of an image since better early warning procedures may be taken if a target can be detected at a far distance.
Omni-directional reflective mirrors are generally used with wide angle video cameras based on Catadioterics framework. One advantage of using a mirror instead of a lens is that a mirror may have a simple structure and much less color aberration than a lens. Hyperbolic mirrors and other omni-directional shaped mirrors are utilized with wide angle video cameras. However, as previously described, the use of such mirrors results in circular images on image sensor chips, thereby having reduced resolution due to using a portion of an image sensor chip and requiring a computing system to unwarp and unwrap an image having a wide angle field-of-view.